1. Technical Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to neighboring cells within a digital wireless communication system. Even more particularly, the present invention relates to managing neighbor-cell lists within the digital wireless communication system.
2. Description of Related Art
In cellular mobile communication systems, a mobile handset communicates with base transceiver stations (“BTS's”) positioned throughout a geographic area. Each BTS broadcasts on a discrete radio channel within a specified coverage area. A multiplicity of BTS sites may produce an array of cells that allows the system to provide radio coverage over a wide geographic area.
During a call, the user of a mobile handset will often move between neighboring cells within the system. As the mobile handset moves from one cell to a neighboring cell, the base station controller (“BSC”) of the serving cell typically transfers the call session to the BTS of the neighboring cell. This transfer of the call is termed “handoff.”
In digital cellular communication systems, such as those utilizing TDMA or CDMA signal transmission protocols, communications between multiple BTS sites and mobile handsets are carried out on radio frequency channels that can carry simultaneous communications. Using these digital protocols, communication sessions are typically performed using digitized voice or data signals that are transmitted as coded bursts or bursts within specified time slots. The coded bursts or time slots corresponding to multiple communication sessions are multiplexed onto the radio channels of the respective cells, and each mobile handset reads and communicates on an assigned decoded channels or assigned time slots on sending and receiving channels. Handoff in digital systems is typically performed using measurements from the mobile handset itself. Known as mobile-assisted handoff (“MAHO”), the method utilizes the mobile handset to periodically monitor and measure radio signals of neighboring BTS sites.
In Global System for Mobile communications (GSM), the mobile station monitors a neighbor cell's Absolute Radio Frequency Channel number (ARFCN) and does calculations for the neighbor cell and the serving cell. The ARFCN's are stored in a Broadcast Control Channel Allocation (BA) list that is broadcast to the mobile station in the idle and dedicated modes. The list of neighbors that the mobile handset measures on a periodic basis is typically contained in a “neighbor-cell list” (hereinafter neighbor list) of the wireless cell that is serving (service cell) the handset. The list may be kept on a database maintained at the site of the serving BTS, retained at a radio network controller (“RNC”) site controlling a plurality of BTS's, or at a centralized mobile switching center (“MSC”). In order to compare measured signals from the neighbor-cells on the neighbor list with the measured signal from the serving cell, the mobile station transmits the results of its measurements to the serving BTS. The serving BTS will in turn forward the measurements to either the applicable RNC's or to the MSC. If the received signal strength of the current channel in the serving cell falls below a threshold or is otherwise lower than the signal strength of a measured channel of a neighboring cell, the MSC or RNC may initiate handoff of the mobile handset's call session to a neighboring cell. Each cell includes an associated neighbor list that includes information on which BSC controls the neighboring cell.
Large neighbor lists in a cellular network result in a large number of handoff candidates. These lists can lead to excessive handoff problems, since the handoff candidates are in the neighbor list, such as dropped calls and loss in overall quality of the cellular network. Excessively large neighbor lists pose a frequency-planning problem as it becomes more difficult to identify available frequencies for neighboring cells without violating frequency-planning constraints.
As a part of preparing the network for an optimum use of cells and frequencies, neighbor lists need to be optimized, i.e., identify and remove unnecessary neighbor-cells. On the other hand, necessary handoff neighbors need to be determined and kept or added to the neighbor list. A current method, well known to those skilled in the art, utilizes peg counters to compute the number of times a handoff occurs between a cell-neighbor-cell (CELL-NCELL) pair. Network planners use this information to determine changes or additions to the system. In many systems facilities for such peg counters do not exist due to the fact that they are memory intensive. For example, in a system with 500 cells, 4000 CELL-NCELL neighbor-cell relationships may exist and a peg counter for each relationship implies 4000 peg counters with memory for each counter. Another drawback to the peg counter method is that the peg counter method cannot be used to identify new neighbor possibilities.
Therefore, It would be advantageous to have a method of optimizing neighbor lists by automatically removing and adding cells to overcome the disadvantages of the existing solutions. The present invention provides such a system and method.